Method of coating zinc base alloys



Patented Oct. 31, 1950 METHOD OF COATING ZINC BASE ALLOYS *Edward A.Kruszynski, St. Louis, and Barclay F.

Smith, Overland, Mo., assignors to Carter Carburetor Corporation, St.Louis,-Mo.,'a corporation of Delaware No Drawing. Continuation ofapplication Serial No. 782,032, October 24, 1947. This applicationSeptember 26, 1949, Serial No. 117,933

'Edward A. Kruszynski, now abandoned.

The invention is particularly concerned with the production of a visibleand iridescent coating on zincand zinc base alloys, for the purpose ofretarding or preventing corrosion and erosion of the metal surfaces uponexposure to the atmosphere or to moisture such as water, salt spray, andthe like. The present coating method is especially suitable for zincbase die casting alloys such as those containing aluminum and magnesium,or aluminum, magnesium and copper,

.and for objects having metal surfaces of zinc or zinc base alloys, aszinc dipped and zinc plated objects.

According to the method of the present invention, there is produced onthe surfaces of objects formed of Zinc or zinc base alloys, and onobjects surfaced with zinc or zinc base alloys, a visible, colored andiridescent coating of a protective, corrosion-resistant character,wherein the coating is in the nature of a thin film such as will notalter in any material degree, the dimensions of the object on which thecoating is formed, nor obstruct or fill up small passageways which maybe provided in th object. In

the treatment, however, the surfaces of all recesses and passageways inthe object, will be coated uniformly and to the same film-like extent asappears on the external or exposed surfaces of the object.

It is a further purpose of the present invention to afiord a coatingsolution or bath which will produce upon the surfaces of metals of theclass hereinabove specified, an insoluble film-like coatingcharacterized by uniformity in thickness, color and iridescence, andwherein the production of the film coating is attained economically andin a minimurn'of time.

The presently improved process is based upon immersion of the metalsurface to be coated, in an aqueous solution or bath consisting ofchromium nitrate acidulated by organic and inorganic acids controlled asto the concentrations thereof in the solution, such as to determine andmain tain the solution at a pH value between 0.8 and 1.3.Advantageously, the acidifying agents em- 5 Claims. (01. 148--6.16)

ployed in the chromium nitrate bath, consist of phosphoric acid as theinorganic agent, and oxalic acid as the organic agent. We have foundthat a visible and iridescent, corrosion-resistant film of uniformcharacter and varying from gray to silver in color, is produced on thesurface of metals in the class hereinabove specified, by immersion ofthe metal in an aqueous solution of this nature, containing per liter ofwater, between and gramsof chromium nitrate, about 13 grams of oxalicacid and about 5 ml. of phosphoric acid, and maintaining the metalimmersed in such solution for a comparatively short time, as between 5seconds and 20 seconds, until the desired color of the film (betweengray and silver) appears. In this solution, the concentration of oxalicand phosphoric acids are relatively closely controlled to theproportions indicated, such as will determine and maintain the solutionat a pH value within the range of 0.8 to 1.3. We have found that the useof the acids specified, in concentrations controlled to affordacidulation of the solution within the pH range given, is quiteimportant to a successful action of the solution in producing thedesired uniform protective film on metal surfaces of zinc and itsalloys.

In practice of the invention according to one mode of carrying out theprocess, an aqueous solution is made up to contain the followingingredients per liter of water:

Chromium nitrate (C1(NO3)3.9H2O), 100 grams Oxalic acid H2CzO4.2H2O, 13grams Phosphoric acid (H3PO4) (90%), 5 ml.

Die castings of zinc base alloy for example, when immersed for about tenseconds in such a solution which is preferably and advantageously heatedand maintained at a temperature of about degrees F.,'have produced onthe surfaces thereof, a chemically bound, insoluble and protective filmwhich is uniformly iridescent and gray or silver in color, and of anature which characterizes the coated castings by a marked resistance tocorrosion upon exposure of the coated metal to the atmosphere ormoisture. Moreover, the silver color of the coating film, as well as itsiridescence, presents a pleasing, decorative appearance, and affordsadditionally, ready identification of objects so coated, indifferentiation from uncoated objects.

In carrying out the present process, before immersion in the coatingsolution, the zinc or zinc base metal objects as die-castings and thelike, are cleaned of dirt, oil and grease deposits on the surfaces to becoated. The surface cleaning step may be carried out in any suitableway, as by immersion of the metal objects in an alkaline bath, or in asolution containing a solvent such as carbon tetrachloride,trichlorethylene or the like, followed by immersion in an alkaline bath.After cleaning, the zinc or zinc base metal surfaces are treated byimmersion in the coating solution of the character and compositionherein specified. While the coating process is not critically dependentupon the temperature of the solution, it is facilitated to someappreciable degree by maintaining the solution at a temperatur of about160 degrees F. More importantly, however, such heated condition of thebath is of great advantage in connection with a following drying stagepresently to be indicated.

Upon completion of the coating stage, the coated metal objects arerinsed preferably in hot water, hot water here being employed in orderto avoid any material cooling of the coated metal which comes from thecoating solution in a heated state consequent to the temperature of thesolution (about 160 degrees F.). Moreover, the use of hot Watermaterially facilitates the rinse operation, by reducing the timerequired to effect a thorough rinse of the coated metal.

Following the rinse step, the hot, coated metal is dried in any suitablemanner, as by subjecting the metal to hot air under natural or forceddraft, and at a temperature ranging up to 200 degrees F. or higher.Since at'the outset of the drying step, the coated metal is in a heatedcondition consequent to the approximately 160 degrees F. temperature ofthe coating bath and the hot water rinse, drying of the coated metal andfixation of the coating film thereon are effected in a relatively shorttime. When so dried, the coating film is set so completely that it doesnot require aging before handling or subjection of the coated metal toabrasion.

While a specific formula of coating bath has been given and certainpossible variations therein indicated, it will be understood that wehave not attempted to indicate all the variations permissible within thegeneral principles set out in theforegoing description.

We claim:

l. The method of producing a visible, colored protective coating on asurface of metal of the class consisting of zinc and a zinc base alloywhich comprises immersing the metal surface for from 5 to 20 seconds ina solution consisting of water, from to grams of chromium nitrate perliter of water, and acidifying agents consisting of oxalic acid andphosphoric acid in proportions of two to three parts oxalic acid to onepart phosphoric acid, torender and maintain the solution ata pH valuebetween .08 and 1.3.

2. The method according to claim 1 wherein the said acidifying agentsare proportioned in the solution as follows: about 5 ml. of phosphoricacid per liter of solution and about 13 grams of oxalic acid per literof solution.

3. The method of producing a visible, colored protective coating on asurface of metal of the class consisting of zinc and a zinc base alloy,which comprises immersing the metal surface for from 5 to 20 seconds ina heated solution containing per liter of solution from 70 to 130 gramsof chromium nitrate, about 13 grams of oxalic acid, about 5 ml. ofphosphoric acid, and the balance Water.

4. The method of producing a visible, colored protective coating on asurface of metal consist.- ing of zinc and a zinc base alloy whichcomprises immersing the metal surface for about 10 seconds in anacidulated solution containing per liter of solution about 100 gram ofchromium nitrate, about 13 grams of oxalic acid, about 5 ml. ofphosphoric acid, and the balance water.

5. The method according to claim, 4 wherein said solution is maintainedat a temperature of about degrees F.

EDWARD A. KRUSZYNSKI. BARCLAY F. SNJITH.

REFERENCES CITED UNITED STATES PATENTS Name 7 Date Thompson Aug. 17,1943 Number

3. THE METHOD OF PRODUCING A VISIBLE, COLORED PROTECTIVE COATING ON ASURFACE OF METAL OF THE CLASS CONSISTING OF ZINC AND A ZINC BASE ALLOY,WHICH COMPRISES IMMERSING THE METAL SURFACE FOR FROM 5 TO 20 SECONDS INA HEATED SOLUTION CONTAINING PER LITER OF SOLUTION FROM 70 TO 130 GRAMSOF CHROMIUM NITRATE, ABOUT 13 GRAMS OF OXALIC ACID, ABOUT 5 ML. OFPHOSPHORIC ACID, AND THE BALANCE WATER.